If I measure the land diameter in a S&W barrel with a pin gage, is there a standard depth in the grooves to then determine the groove diameter? With the odd number of grooves, it is difficult to measure a slug. An alternate test would be useful.

I guess I didn't ask the question clearly. Is there any information available that states the "standard" groove depth (or land height) in S&W revolver barrels? Does this measurement change with different models, years, or is it supposed to be constant?

There is a "nominal" dimension, but the whole point of measuring YOUR barrel is to see what it actually is. Does it seem worthwhile to measure bore and then estimate groove with a "nominal" dimension? What does that really tell you?

Well, as demonstrated by the picture above (micrometer) and the methods used to wrap the slug to measure one with an odd number of grooves, it is difficult to get a precise measurement. What I was hoping for was a comparative based on the internal bore (land) measurement. Without buying the expensive micrometer, I am not sure I have the correct diameter by cutting up a beer can and wrapping the slug. Using that method in my 44 Spl I come up with .429 for a groove diameter but using a pin guage thru the barrel I get .416 which is the bore, or land, diameter. A groove depth of .0065 seems like it would be too much so I am concerned I have an error. The obvious place to look is the least precise measurement, the groove diameter of .429. That is the background to my question.

I always feel that slugging the bore is a very valuable step. I oil the bore well and use pure lead (muzzleloader swaged balls) to do this. I feel for tight spots as the ball passes through the bore (like where the barrel passes through the frame.

Next I take the lead slugs and check them in the cylinder throats. If they are larger than the throats and you can't press them through with light finger pressure, you are going to have problems. It means your throats are too small for the groove diameter. If they do pass through, use your pin gages to determine the throat diameter and use that for your bullet selection.

I have never read where anyone was successful in trying to use pin gages in the barrel and then by using a land height to determine the groove diameter. But you might be the first. When I played around with it, I just couldn't get an accurate measurement.

And if this doesn't discourage you, send them to Matt Dardas. He offers a free service to measure the groove diameter of your slugs for FREE! How can you beat that?

In a revolver the fit of the bullet to the cylinder throats is more important than knowing the groove diameter. Simply slug the throats and size to that diameter.

If the cylinder throats are smaller than groove diameter, and this is not uncommon, it doesn't matter what size you make your bullets they will be undersized before they even get to the barrel!

Do this. Slug the barrel and then try to pass the slug through the throats. If it passes easily, or with slight pressure, size your bullets to fit the throats. If it will not pass relatively easily through the throats (Use a new bore slug for each charge hole.) then the cylinder throats need to be enlarged until the bore slugs do pass with light pressure. It doesn't do you one d****d bit of good to know the barrel groove diameter if you don't know the other dimensions too!

Note: Never in the process do you have to measure the barrel slug as how it fits the cylinder is all that matters!

I second (or third, or ???) that the cylinder throat diameter is the only thing you are ever going to be able to come to a conclusion about, so you should avoid buying bullets that are going to get swaged coming out of the cylinder.

The first time I went poking around trying to measure the bore of a revolver I was gobsmacked... The damn thing, that shoots like a laser, had an insane constriction where the barrel was screwed into the frame. After discovering the SAME THING on two more infallible shooters, I quit.

As long as the bullet passes into the barrel intact, it will (and of course I'm only talking about lead bullets that are getting a good solid push to a decent velocity) be the right size at the right time.

Speaking of Matt Dardas, his bullets are the only ones that have got me out to 150 yards from a 1911 with accuracy and punch. I'd swear that the low melting point lube is half the trick, it's a little like the slime that's on 22lr match bullets.